Centrifugal compressors have wide application in many industries. In cryogenic separation plants compressors are used to separate air into its constituent parts, air is compressed by a multi-stage centrifugal compressor and then cooled to a temperature suitable for the distillation of the air. The air, after having been cooled is rectified in a distillation column to produce nitrogen, oxygen and argon products. In such plants, centrifugal compressors are also employed as product compressors to compress product nitrogen gas and product oxygen gas.
Although in any compression application, it is possible to compress the gas in a single stage, in many industries, including the cryogenic air separation industry, it is more common to compress the gas in sequential compression stages, particularly when the discharge pressure is higher than one and half times the inlet pressure. The reason for this is as the gas is compressed, its temperature rises and the elevated gas temperature requires an increase in power to compress the gas. Where the gas is compressed in stages and cooled between stages, the compression power requirement is reduced due to closer to isothermal compression compared to compression without interstage cooling. In a typical compressor installation, each stage uses a centrifugal compressor in which gases entering an inlet to the compressor are distributed to a vaned compressor wheel known as an impeller that rotates to accelerate the gas and thereby impart the energy of rotation to the gas. This increase in energy is accompanied by an increase in velocity and a pressure rise. The pressure is recovered in a static vaned or vaneless diffuser that surrounds the impeller and functions to decrease the velocity of the gas and thereby increase the pressure of the gas.
The individual compressors of the compression stages of a multi-stage compressor can be driven by a common driver, such as an electric motor driving an integral gearbox. However, in one type of compressor assembly, a compressor stage is directly connected to the electric motor such as a permanent magnet electric motor without gearing. The direct coupling of the compressor and the electric motor overcomes the inefficiencies inherent in a gear box arrangement in which thermal losses occur within the gearing between the electric motor and the compressor. Such a direct coupling is known as a direct drive compressor assembly where both electric motor shaft and the impeller rotate at the same speed. Typically such electric motors are capable of variable speed operation. A directly driven compressor can thereby be operated to deliver a range of flow rates through the compressor and a range of pressure ratios across the compressor by varying the driver speed.
Direct drive compressor assemblies can be configured, for example, by installing a compressor impeller on one end of a shaft of an electric motor. The compressor impeller and the motor rotor rotate at the same speed. It is also possible that a direct drive compressor assembly contains two or more compressor impellers driven by a common motor, and installed on opposite ends of the same shaft of the electric motor. It is also possible that a single motor drives two compressors connected at one end of the electric motor shaft. Several permutations and combinations are possible for configuring a direct drive compressor assembly depending on the number of compressor impellers, motor, and any other rotatable driver or driven device installed on the common shaft.
A prevalent failure mode in a direct drive compressor assembly is the impeller which may experience a crack and then lose a portion of blading. The loss of blading usually creates very significant unbalance forces which must be reacted by the rotor and its bearing system. The unbalance in the loading on the motor shaft will be produced because the mass of the rotating impeller is not equally distributed in a radial direction of the impeller. In other words, a force will be produced due to the unequal distribution of mass and the rotation of the impeller that will act at right angles to the shaft. As a result of such unbalance loading, the motor shaft will experience additional forces and moments with the support bearings having to react to these additional unbalance loads which can lead to a failure of radial bearings used in the electric motor. These bearings can be oil lubricated bearings or foil or electromagnetic bearings that support the motor shaft both in rotation and in an axial direction and back-up bearings that support the motor shaft in case of a failure of the foil or electromagnetic bearings. In any event, the failure of such bearings will lead to complete destruction of the electric motor.
Aside from impeller blade cracking, other abnormal operating conditions that can create very significant unbalance forces include impeller cracking in non-bladed areas, erosion of impeller material, deposition of fouling products or foreign debris in the impeller, and the unintended loosening of parts on the impeller end of the common rotating shaft. This list is not meant to be a comprehensive list and those experienced in the art would recognize other abnormal operating conditions that can create very large unbalanced forces and moments that if remain unchecked could result in permanent deformation of the motor shaft.
In the prior art, break-away assemblies have been used in various devices using compressors to contain structural failures of an impeller and to prevent damage to associated equipment. For instance, in U.S. Pat. No. 7,001,155, a supercharger is provided in which a compressor, connected to an exhaust gas driven turbine, is provided with an impeller that has a threaded bore extending through the impeller hub to engage with a threaded end of a drive shaft. The impeller bore is provided with an enlarged portion that produces a thin wall section of the impeller hub that will fracture before the threads within the bore will strip. This ensures that any failure of the impeller will leave the hub intact and connected to the shaft to contain the failure and thereby to prevent damage to the engine. In U.S. Pat. No. 5,443,372, an automotive air conditioning compressor has coupling plate members that include portions which may be easily broken or fractured under a predetermined amount of torque or other applied mechanical force to also prevent engine damage.
In a compressor assembly such as in an air separation plant, the electric motor driving the compressor is typically a very powerful device consuming perhaps 0.1 to 50 megawatts of power. Consequently, the torque transmitted by the electric motor to the compressor impeller requires a coupling between the motor shaft and the impeller that is sufficiently robust to allow such torque to be transmitted. Consequently, prior art breakaway solutions, such as have been discussed above, are not applicable to industrial compressor applications that involve high levels of power transmission between the motor shaft and the impeller.
As will be discussed, unlike the prior art breakaway solutions, the present invention provides a coupling that will allow the impeller to fail before the electric motor shaft from being damaged as a result of an impeller failure and resulting very large unbalance loads.